Field
Aspects of the present invention generally relate to methods and devices for positioning and/or rotating a substrate during semiconductor device fabrication in a processing chamber.
Description of the Related Art
Integrated circuits are complex devices that can include millions of transistors, capacitors and resistors on a single chip. Chip design continually requires faster circuitry and greater circuit density and thus demands increasingly precise fabrication processes.
In some fabrication processes, such as ion implantation, film layers on the substrate develop high levels of internal stress. In order to relieve the stress and control the film properties and uniformity, the film is subjected to a thermal process, such as annealing. Rapid thermal processing (RTP) chambers subject substrates to highly controlled thermal cycles, such as heating a substrate to over 1000° C. in less than ten seconds. RTP relieves stresses in film layers and can also be used to modify film properties such as changing the density or the electrical characteristics of the film.
However, RTP processes can cause uneven heating across the surface of the substrate, especially where the substrate is in contact with other components such as a substrate support or a support ring. For example, in many wafer (substrate) handling systems, the wafer handler may comprise an assembly that comes in contact with the wafer. This is beneficial in cases where the handled wafer needs to be rotated, because the position and rotational speed can be controlled using features that are designed into the wafer handler or rotor. But problems arise from uneven heating of the wafer due to the wafer contact.
Accordingly, systems have been developed to support, position and rotate a wafer during an annealing process without direct contact with the wafer. U.S. Pat. Nos. 8,057,602 and 8,057,601, assigned to Applied Materials, Inc., describe devices and methods for floating, positioning and rotating a wafer on a thin layer of air, and are herein incorporated by reference. Because the wafer is no longer in direct solid contact with other system components, precise sensors and control systems are needed to monitor and control both the position and the rotation of the wafer.
For no-contact wafer positioning, optical sensors have been used to monitor the position of the wafer's outer edge. However, optical sensors may suffer reliability problems when exposed to harsh chamber conditions, due to high temperature exposures (in some cases over 1000° C.) or due to process gases leaving deposits on the optical components. Additionally, a reliable solution is needed for tracking and controlling wafer rotation. In the past, a notch on the outer edge of a wafer has been used to orient wafers in processing chambers. However, if notched wafers are used in no-contact wafer positioning systems, errors are introduced whenever the notch rotates through the field of view of an optical sensor. Whenever an optical sensor “sees” the notch, a system controller tries to “re-center” the wafer, and mistakenly moves the wafer off-center. When that particular sensor sees the true edge again, another correction is prompted. Therefore, a need exists for improved devices and methods for positioning and rotating wafers without direct solid contact.